1. Field of the Invention
The present invention relates to a plasma display apparatus and an image processing method thereof, and more particularly, the present invention relates to an improved plasma display apparatus and an image processing method thereof which can enhance a gray level representation capability.
2. Description of the Background Art
In general, in the plasma display apparatus, a wall formed between a front substrate and a rear substrate constitutes one unit cell, each cell is filled with a main discharge gas such as neon (Ne) or helium (He) and a mixture gas (Ne+He) and an inert gas containing a small quantity of xenon. When the discharge gas is discharged by the high-frequency voltage, the inert gas generates vacuum ultraviolet ray and the fluorescent substance existed between the walls emits light, and so the image is embodied. The plasma display apparatus as described above has been in the limelight as the next generation display apparatus since it is possible to make the plasma display apparatus having a thin and light structure.
FIG. 1 is a view illustrating a method for embodying the image in the conventional plasma display apparatus.
In the plasma display apparatus, as shown in FIG. 1, one frame period is divided into a plurality of subfields (the frequency of discharge in each subfield differs from those in the others), the plasma display panel emits the light in the subfield period corresponding to a gray level value of an image signal to be input, and so the image is embodied.
Each subfield is divided into a reset period for exciting uniformly the discharge, an address period for selecting the discharge cell and a sustain period for embodying the gray level according to the frequency of discharge. For example, in the case that the image with 256 gray levels is displayed, the frame period (16.67 ms) corresponding to 1/60 second is divided into eight subfields.
In addition, each of eight subfields is divided into the reset period, the address period and the sustain period again. Here, the sustain period is increased in each subfield at the rate of 2n (n=0, 1, 2, 3, 4, 5, 6, 7). Since the sustain periods in the subfields differ from each other as described above, the gray level of the image can be embodied.
FIG. 2 is a graph for comparing the luminance characteristic of the plasma display apparatus with the luminance characteristic of the cathode ray tube.
In the cathode ray tube and the liquid crystal display device, the light to be displayed according to the input video signal is controlled in an the analog manner to represent the desired gray level, and so the cathode ray tube and the liquid crystal display device generally indicate a non-linear luminance characteristic. In the plasma display apparatus, contrary to the cathode ray tube and the liquid crystal display device, the number of the light pulse is modulated by using a matrix array of the discharge cells which can be turned on/off to represent the gray level, and so and the plasma display indicates a linear luminance characteristic. Such method for representing the gray level of the plasma display apparatus is called as the PWM (pulse width modulation) method.
At this time, since the brightness characteristic vs the display current is in proportion to a 2.2 multiplier, the display apparatus such as the cathode ray tube sends the signal corresponding to a reciprocal of 2.2 multiplier of an external input image signal such as the broadcasting signal. Accordingly, there is a need to inverse gamma correct the external input image signal in the plasma display apparatus indicating a linear brightness characteristic.
FIG. 3 is a graph showing an inverse gamma correction in the conventional plasma display apparatus.
In FIG. 3, the target luminance indicates the ideal result to be obtained by the inverse gamma correction, the real luminance indicates a measured luminance value represented as a result of the inverse gamma correction and the PDP luminance indicates the luminance value of 3 or less which is measured in the state that the inverse gamma correction is not performed.
As shown In FIG. 3, the target luminance is represented as one of the luminance values, each of which has the gray level of 61 steps (0 through 60). On the contrary, the real luminance is represented as eight luminance values, each of which has one of the gray levels of 61 steps (0 through 60). Accordingly, when the inverse gamma correction process is performed in the plasma display apparatus, a sufficient gray level representation can not be obtained in a dark area, and so there is a problem that the contour noise in which the images are lumped together is appeared.
In order to enhance the insufficient gray level representation capability of the plasma display apparatus, a half tone method such as a dithering method and an error diffusion method and the like has been used.
First of all, in the error diffusion method, fraction generated when the gray level value of the corresponding pixel is quantized, that is, an error has influence on the adjacent pixels so that the correction to an error to be discarded is spatially solved. An error diffusion coefficient to the adjacent pixel is set constantly, and so such error diffusion method is repeated to each line and each frame. Accordingly, there is a problem that the same error diffusion pattern is formed on the entire screen due to the constant error diffusion coefficient.
Next, the dithering method is the method for judging whether a carry is generated or not by comparing the gray level value of each pixel with a specific threshold of a dither mask. That is, the dithering method is the method for enhancing the insufficient gray level capability by turning on the pixel in which the carry is generated and turning off the pixel in which the carry is not generated. Such dithering mask uses a plurality of dither masks on which constant patterns are formed. Accordingly, there is a problem that the patterns of the dither mask are displayed on a screen due to repeated use of the dither mask.
In order to overcome the above problem of the error diffusion method and the dithering method and enhance the gray level capability, the error diffusion method is used together with the dithering method as shown in FIG. 4.
FIG. 4 is a view for illustrating the conventional method for using the error diffusion technique together with the dithering technique.
In the method in which the error diffusion and the dithering are used together, as shown in FIG. 4, a gray level data of the image signal which is already inverse gamma corrected is divided into an integer bit and a fraction bit first and the fraction bit is divided again into a superior bit and a subordinate bit. Then, the error diffusion is performed to the subordinate bit, if the carry is generated, the carry is reflected in the superior bit. Also, the dithering is performed to the superior bit, if the carry is generated, the carry is reflected in the integer bit. At this time, the integer bit is called as the real gray level, the image of the plasma display apparatus is embodied finally by using the real gray level value, and so it is possible to represent the various gray levels.
On the other hand, the method in which the error diffusion and the dithering are used together has a problem that a flicker is generated on the embodied image as shown in FIG. 5.
FIG. 5 is a view illustrating the problem occurred when the conventional method in which the error diffusion and the dithering are used together is applied.
As shown in FIG. 5, since the error diffusion is performed to entire pixels of each frame by means of the adjacent pixel placed at a constant position and a constant error diffusion coefficient, when the dithering to each frame is performed with different dither mask pattern, the patterns are matched with each other, and so the various problems are occurred.
As shown in FIG. 5, for example, pixels of the error diffusion pattern in which carry is generated are matched with pixels of the A typed dithering mask pattern in which carry is generated, and so desired image is embodied, while the pixels of the error diffusion pattern in which carry is generated are not matched with pixels of the B typed dithering mask pattern in which carry is generated, and so desired image is not embodied at all. As described above, if the error diffusion pattern is not matched with the dithering mask pattern, luminance differences of the screen on which the image is displayed are generated up to 50%. Due to such luminance differences, the flicker phenomenon in which the screen is flickered is occurred. Here, dark marks in the error diffusion pattern and dithering mask pattern indicate locations of the pixels in which carry is generated, dark marks in the screen on which the image is displayed indicate locations of the pixels which are turned on/off depending on whether carry is generated or not.
Also, due to the problems as described above, each gray level of R (red), G (green) and B (blue) of the origin image signal is distorted, and so the color of the image to be embodied is changed.